This disclosure relates to electrochemical cells, and, more particularly, to an apparatus for maintaining compression within the active area of an electrochemical cell.
Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, an anode feed electrolysis cell is shown at 10 and is hereinafter referred to as “cell 10.” Reactant water 12 is fed to cell 10 at an oxygen electrode (e.g., an anode) 14 where a chemical reaction occurs to form oxygen gas 16, electrons, and hydrogen ions (protons). The chemical reaction is facilitated by the positive terminal of a power source 18 connected to anode 14 and a negative terminal of power source 18 connected to a hydrogen electrode (e.g., a cathode) 20. Oxygen gas 16 and a first portion 22 of the water are discharged from cell 10, while the protons and a second portion 24 of the water migrate across a proton exchange membrane 26 to cathode 20. At cathode 20, hydrogen gas 28 is formed and is removed for use as a fuel. Second portion 24 of water, which is entrained with hydrogen gas, is also removed from cathode 20.
Another type of water electrolysis cell that utilizes the same configuration as is shown in FIG. 1 is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell also utilizes the same general configuration as is shown in FIG. 1. Hydrogen gas is introduced to the hydrogen electrode (the anode in the fuel cell), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in the fuel cell). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, a hydrocarbon, methanol, or any other source that supplies hydrogen at a purity level suitable for fuel cell operation. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water.
Conventional electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a membrane electrode assembly (hereinafter “MEA”) defined by a cathode, a proton exchange membrane, and an anode. Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either one side or both sides by flow field support members such as screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA.
Referring to FIG. 2, a conventional electrochemical cell system suitable for operation as an anode feed electrolysis cell, a cathode feed electrolysis cell, or a fuel cell is shown at 30 and is hereinafter referred to as “cell system 30.” Cell system 30 includes the MEA defined by anode 14, cathode 20, and proton exchange membrane 26. Regions proximate to and bounded on at least one side by anode 14 and cathode 20 respectively define flow fields 32, 34. A flow field support member 36 is disposed adjacent to anode 14 and is retained within flow field 32 by a frame 38 and a cell separator plate 40. A flow field support member 42 is disposed adjacent to cathode 20 and is retained within flow field 34 by a frame 50 and a pressure pad separator plate 44. A pressure pad 46 is disposed between pressure pad separator plate 44 and a cell separator plate 48. The cell components, particularly frames 38, 50 and cell separator plates 40, 48, are formed with the suitable manifolds or other conduits to facilitate fluid communication through cell system 30.
A pressure differential often exists within the cell system and particularly across the cell. Such a pressure differential may cause variations in the pressure distribution over the surface area of the MEA. In order to compensate for the pressure differential and while maintaining intimate contact between the various cell components under a variety of operational conditions and over long time periods, compression is applied to the cell components via pressure pad 46. However, because pressure pad 46 is generally fabricated from materials incompatible with system fluids and/or the material from which the cell membrane is fabricated, pressure pad 46 is oftentimes separated from the active area of the cell by pressure pad separator plate 44 and/or enclosed within protective casings (not shown).
While existing pressure pads are suitable for their intended purposes, there still remains a need for improvements, particularly regarding the compression of the components in the electrolysis cell and support of the MEA, particularly at high pressures. Therefore, a need exists for a pressure pad that is compatible with the cell environment, and that provides uniform compression of the cell components and support of the MEA, thereby allowing for the optimum performance of the electrolysis cell.